1. Field of the Invention
The present invention relates to a method and an apparatus for eliminating static electricity to prevent the trouble caused by static electricity.
More particularly, the present invention relates to a method and an apparatus of the ion supply type for creating positive and negative ions by corona discharge and spraying them onto a charged body whose load density is in imbalance to neutralize the load of the charged body and eliminate static electricity from the charged body.
Still more particularly, the present invention relates to a static electricity eliminating method an apparatus in which a new electrode arrangement is used to hold for a long time the balance of these positive and negative ions created.
The method and apparatus of the present invention can be employed to eliminate static electricity in type, offset and gravure printings, for example, silk screen and film processes, assembling and punching plastic and paper bags and the trimming plastics and papers to enable them to be used for wrappings and in the course of processing fibers, and also to eliminate static electricity from flowing grains as well as electronic components.
2. Description of the Prior Art
Most electronic components are usually broken into deficient ones by static electricity of about several hundreds volts. Further, static electricity, higher than several tens of kilovolts, is created on surfaces of plastic products and films. Therefore, they cause various kinds of troubles such as causing dust to stick to them. If even a piece of dust is stuck to a substrate by static electricity at the time when a semiconductor circuit is printed on the substrate, for example, the whole of the substrate will be regarded as a deficient one and cannot be used. Troubles like this caused by static electricity can be found everywhere in commercial, industrial and common lives and thus, it can be seen that static electricity is quite troublesome. Various kinds of measures have been thus employed to eliminate troubles caused by static electricity.
The static electricity eliminating means of the corona discharge type is one of these measures. The eliminator means of the corona discharge type can take one of several forms. One of them now used is of the selfdischarge type. In the case of the eliminator means of the self-discharge type, however, it is difficult to completely eliminate static electricity from charged bodies. Another method has been thus developed comprising creating ions by corona discharge and spraying them onto the charged body whose load is reverse in polarity to that of the ions to neutralize the load of the charged body or balance the load density thereof. This method is grouped in the alternating- and direct-current systems. It is the alternating-current system that is now more widely used.
The eliminator means of the alternating-current (AC) type is intended to convert the commercial power of AC 100V (Japanese standard) to a high AC voltage of 5-10Kv by means of a converter and/or transformer and the like, apply it to the discharge electrode (or discharge needle) to create corona around the electrode, and create positive and negative ions alternately due to this corona phenomenon to supply them to the charged body. As the result, static electricity carried by the charged body is neutralized by ions sprayed, and eliminated from the charged body.
The above-described eliminating method and means of the AC type is disclosed in Japanese Utility Model Publications Sho 34-13629, 38-18505, 40-16334, 41-11945 and 55-15278, Japanese Utility Model Pre-Publications Sho50-32967, 50-131964 and 51-39476, Japanese Patent Publication Sho 43-16026, and Japanese Patent Pre-Publications Sho 58-225600 and 59-12600.
In the case of the static electricity eliminating method of the AC type, static electricity carried by the charged body can be eliminated whichever polarity the load of the charged body may have. However, positive and negative ions created combine partly with one another again. Because positive and negative ions are alternately created through the same corona discharge needle, ions whose polarities are different from one another are liable to be neutralized in the vicinity of the corona discharge needle. Therefore, the amount of ions useful for eliminating static electricity is small, which means that the ability of eliminating static electricity is low in the case of the eliminating method of the AC type.
Further, ions created around the electrode interfere with one another to lower the effect of eliminating static electricity as frequency is made higher. As frequency is made lower, however, it becomes more difficult to uniformly eliminate static electricity carried by the charged body. In other words, the effect of eliminating static electricity cannot be expected.
Furthermore, the polarity of current changes every cycle and positive and negative ions are thus alternately and intermittently created. Therefore, the amount of ions becomes sometimes short, depending upon the load density of the charged body to be processed, thereby lowering the effect of eliminating static electricity.
There has been well known the eliminator means of the direct-current (DC) type, which is intended to apply high DC voltage to the discharge electrode to create ions of a single polarity. The eliminator means of the DC type can thus eliminate the abovementioned drawbacks of the one of the AC type eliminating method.
According to the eliminator means of the DC type, a lot of ions useful for eliminating static electricity can be generated to enable the eliminator means to have a quite higher capacity of eliminating static electricity. However, ions having only a single polarity, positive or negative, can be generated and when the polarity of the load carried by the charged body is same as that of ions created,.therefore, the eliminator means cannot be applied to this charged body. This is the reason why the eliminator means of the amphoteric discharge type has been provided to use DC electrodes as discharge ones to generate positive and negative ions at the same time.
Even in the case of the eliminator means of the amphoteric, discharge type, it is necessary that the flow of ions be balanced depending upon the polarity and load density of the charged body, and this technique of balancing the flow of ions is disclosed in Japanese Patent Pre-Publication Sho 57-11499.
The inventor of the present invention has studied the technique of balancing the flow of ions and found that the effect of holding the flow of ions balanced lasts for a quite short time, that is, two months or less in the case of the conventional apparatus.
After studying the cause of this drawback from various viewpoints, the inventor has found that this drawback is caused by the electrode through which corona is generated. More specifically, a needle-like metal having a sharp tip is used as the conventional corona discharge electrode and when corona discharge is repeatedly carried out through this electrode, load particles adhere and deposit on the stepped shoulder of the electrode which is adjacent to the tip of the electrode, thereby causing electrolytic corrosion at the stepped shoulder of the electrode. This is the reason why the effect of holding the flow of ions balanced lasts only for a short time.
This was not seen in the case of the eliminator means of the AC type. The reason is supposed to reside in that alternately applied positive and negative ions repel load particles, whichever polarity the load particles may have, to prevent the load particles from depositing on the electrode. The load particles deposited because of corona discharge conducted through the electrode for a long time add electrochemical effect to the electrode and cause the electrode to be electrolytically corroded. This electrolytic corrosion is supposed to make quite short the lasting of the effect of holding the flow of ions balanced. In the case of applying DC voltage, no force acts to prevent the load particles from depositing on the electrode.